Vapor generators of the kind in which a fuel-air mixture is combusted in the direct presence of feed water to produce a useful mixture of steam and non-condensibles are known. See the vaporizers shown in U.S. Pat. No. 3,980,137 and British Pat. No. 283,290. Other similar equipment is shown in U.S. Pat. Nos. 1,483,917; 2,168,313; 3,101,592 and 3,449,908.
One difficulty which has been encountered in vaporizers in the past is that of high carbon monoxide content in the product vapor, which is objectionable for many applications and dangerous for some of them. High carbon monoxide production is traceable to incomplete combustion, which is in turn traceable in part to difficulties in maintaining a stable lean flame, and in part to excessive quenching of the flame through direct radiative and convective contact between the flame and the feed water.
Another disadvantage of past vapor generators is encountered especially when they are operated at low pressures. Conventionally, as shown in my prior co-pending application Ser. No. 907,694, vapor generators are started by spark ignition. A spark plug is provided, and it is activated for starting. After spark activation, flow of fuel and air to the combustion chamber is commenced, and the spark strikes a flame in the flowing mixture. However, it may happen that a combustible mixture partially fills the combustion chamber before combustion begins. In such case, a small explosion occurs in the combustion chamber when the flame is struck. The explosion is characterized by a rapid rise in temperature and pressure.
When the generator is one in which the fuel and air supply pressures are relatively high (e.g. 100 psig) compared to the combustion chamber pressure (e.g. 20 psig), sonic velocity is attained in the air and fuel delivery system, and the small explosion upon ignition causes little or no problem.
But when the generator is one in which the fuel and air supply pressures are relatively low (e.g. 10 psig) compared to combustion chamber pressure (e.g. 5 psig), the pressure pulse accompanying the small explosion may cause a change in the fuel/air ratio or even momentarily stop fuel and/or air flow. This results in undesirable rough combustion. Combustion proceeds by a series of small explosions, instead of in a smoothly established flame.